A modulator typically includes two conductive regions with a modulation region between them, through which an optical waveguide passes. In operation, bits can be imposed on the light passing through the modulator's waveguide by applying a voltage that has either of two values. One value makes the modulator more transparent, while the other value makes it more opaque. The ratio of the optical power exiting the modulator in these two states, for given values of voltage, is called an extinction ratio. The maximum rate at which bits can be imposed on the light is called a bit rate. At high bit rates, the voltage applied to the modulator varies very rapidly, so the performance of the modulator becomes increasingly important as the modulation frequency increases. It is desirable for modulators to have both the high extinction ratio and high bit rate.
Conventionally, a modulator is operated as a simple lumped device. An input transmission line with a characteristic impedance Z0 typically connects to the modulator, which is directly shorted by a load resistor with resistance Rload, which is typically equal to Z0. Unfortunately, this is a non-ideal approximation to the desired matched load situation, because the parallel combination of the modulator's lumped parasitic and Rload terminate the line, not just Rload alone.
As transmission frequencies increase, e.g., to data rates of 40 gigabits per second (Gbps) and beyond, the impedance of this parallel combination typically gets lower and lower due to the fact that impedance of a capacitor is inversely proportional to frequency and behaves increasingly like a short as frequency increases. Consequently, at increased frequency, the load at the end of the input line typically becomes more and more poorly matched to the line. This in turn causes increased reflection back on the input line, and decreased signal at the modulator. Both of these effects limit performance of the modulator at high frequency and hence limit bit rate.
These limitations on high frequency performance may be alleviated by making the modulator shorter along the direction of the optical waveguide. The parasitic capacitance of the modulator would then be reduced, approximately in proportion to length. This may improve the high frequency performance. However, it would also decrease the modulator's extinction ratio, which is approximately proportional to its length. Thus, there typically is an unfortunate trade-off between these two desirable aspects of modulator performance.